The invention relates to an electromagnetic actuator for moving a valve in translation so as to bring it alternately into an open position and into a closed position. A major application of the invention lies in controlling the valves of internal combustion engines, with spark ignition or compression ignition.
At present, the valves on most internal combustion engines are actuated by a cam shaft driven by the engine. The opening and closing velocities of valves controlled by a cam shaft are small when the engine is running slowly, in particular on starting, which is not favorable to filling the combustion chambers.
Proposals have also been made (U.S. Pat. No. 4,614,170) for an electromagnetic actuator that enables the above-mentioned drawbacks to be reduced, the actuator having a ferromagnetic armature fixed to the stem of the valve, resilient return means for holding the valve at rest in a middle position between its fully open and closed positions, and electromagnetic means for moving the valve in both directions in alternation. The electromagnetic means described in document U.S. Pat. No. 4,614,170 have a first ferromagnetic core electromagnet placed on one side of the armature which, when excited, attracts the armature causing it to tend to close the valve, and a second electromagnet placed on the other side of the armature which, when excited, tends to bring the valve into its fully open position.
The valve and spring assembly constitutes an oscillating system excited by periodically powering the electromagnets in alternation. The electromagnet acting on the armature in the valve-opening direction begins to be powered when the armature is approaching a location in a hich it sticks to the core of the electromagnet.
The invention seeks in particular to provide an electromagnetic actuator that satisfies practical requirements better than those in the prior art, in particular by being of reduced size and requiring fewer connections.
For this purpose, the electromagnetic means comprise a single coil mounted on a ferromagnetic circuit of structure such that, in combination with the armature, it presents two stable paths for magnetic flux, each corresponding to an airgap of small size (generally no gap).
One of the configurations corresponds to the valve being fully open, and the other to the valve being closed.
In its initial state, in a middle position, the armature generally presents position or magnetic circuit unbalance because the direction in which it is attracted when the coil is first powered is predetermined. This unbalance can be provoked deliberately. For example, when the resilient return means are constituted by two springs placed on respective sides of the armature, the two springs can be such as to give the armature at rest a position in which the force that results from powering the coil acts in a determined direction and that they present the same potential energy in compression both in the closed position and in the fully open position.
An advantageous manner of unbalancing the magnetic forces acting up and down is to cause the flux in the central portion to be asymmetrical by acting on a lamination notch profile and/or on an armature profile.
To provide asymmetry, the armature can have an axial projection. Another manner of creating asymmetry consists in giving the poles of the ferromagnetic circuit and the armature shapes such that the contacting surfaces in both stable paths are different.
Since it has a single coil only, the actuator is more compact than prior actuators. Its electrical circuit and control are simpler and less expensive.
The above characteristics and others that are advantageously used in association with the preceding characteristics, but which can be used independently, will appear more clearly on reading the following description of particular embodiments given as non-limiting examples.